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A simple Spotmatic SPII battery test
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PostPosted: Thu Jun 17, 2010 12:05 pm    Post subject: A simple Spotmatic SPII battery test Reply with quote
This morning I received an adapter from the "Small Battery Company", which converts the 1.5V output of a modern watch battery to the 1.35V that the Spotmatic SPII light meter was designed for. First of all, I am very happy with the metering of my camera as it is, with the E387S batteries, but I would like to see if it could be improved even more. Besides, type 377 watch batteries are very easy to find (though the E387S batteries can be found on Ebay as well).

I tested indoors in low light, indoors in the living room, and outdoors (sunny day, some clouds). In all cases I used the same flat surface (a large sheet of greenish paper) as a test surface.

I have used my SPII with the E387S battery and the regular battery cover; the SPII with the 377 battery and the H-B adapter, and my Canon 20D. The SPII was fitted with a Tokina 3.5/200mm lens. The 20D was fitted with a 70-210/3.5-4.5 lens, set to approx. 120mm. The metering mode of the 20D was set to central weighted average.

A bit to my surprise, there was none or just very little difference between the three configurations. In none of the cases there was much more than 0.5 stop between the readings of the cameras, which I think is good enough in practice.

Indoors, low light
SPII, E387S: 200ASA / 1:3.5 / 1/8
SPII, H-B adapter: 200ASA / 1:3.5 / 1/8
20D: iso200 / 1:5.0 / 1/8

Indoors, living room
SPII, E387S: 200ASA / 1:4.5 / 1/30 (estimated, between f/3.5 and f/5.6)
SPII, H-B adapter: 200ASA / 1:4.5 / 1:30 (estimated, between f/3.5 and f/5.6)
20D: 200ASA / 1:4.5 / 1/30

Outdoors
SPII, E387S: 200ASA / 1:14 / 1/250 (estimated, between f/11 and f/16)
SPII, H-B adapter: 200ASA / 1:14 / 1/250 (estimated, between f/11 and f/16)
20D: iso200 / 1:14 / 1/250

This was no scientific testing, but in real life, one is not photographing under scientific conditions either. But it shows that, at least for my SPII, it makes very little difference whether I use a E387S battery or a 377 battery plus adapter. Besides, it shows that the SPII and the 20D are very much in agreement, despite the old age (thus possible aging) of the SPII CdS cell.

Actually, if I had known this on beforehand, I had not spent money on the H-B adapter, even if it has the convenience of using regular watch batteries.

Please beware: I have only tested this with my own Spotmatic, with a single E387S cell, a single adapter plus battery, and a single Canon 20D as comparison.


PostPosted: Thu Jun 17, 2010 12:25 pm    Post subject: Reply with quote
Yeah, but thanks anyway ... Very Happy


PostPosted: Thu Jun 17, 2010 3:26 pm    Post subject: Reply with quote
This is what happened to me using a 1.5v battery:



PostPosted: Thu Jun 17, 2010 5:00 pm    Post subject: Reply with quote
peterqd wrote:
This is what happened to me using a 1.5v battery:


Yes, I believe you. But I don't understand how the voltage sensitivity seems to vary so much between individual spotmatics. Mine meters great with a E387S battery, and I read the same from other users. But there are also people who tell, and demonstrate, that metering is off with a higher voltage than intended.

This is what happens with my pictures when I meter using a 1.5v battery, and my experiment from today suggests that it will be similar with a 1.35v battery:



PostPosted: Thu Jun 17, 2010 5:49 pm    Post subject: Reply with quote
From my reading, the original Spotmatic meter circuit is an 'unbalanced' design wherein battery voltage affects meter reading, however Asahi Pentax used a 'balanced' circuit, unaffected by battery voltage, in SPII, SPF, ES, and ESII.

Then an engineer posted an article online showing how battery voltage does affect meter reading in Asahi's 'balanced' circuit. (I wasn't convinced; need to take another look)

Folks continue to report battery voltage makes a metering difference in their SPII. Maybe some SPII have the balanced circuit and some don't. Serial numbers...

SPII people with access to a good quality bench power supply capable of varying from 1.35 to 1.50 VDC output could power an SPII and tell us if the meter reading changes when voltage is changed. If different folks get different results, the serial numbers list should show which come before & after a circuit change.


PostPosted: Thu Jun 17, 2010 8:41 pm    Post subject: Reply with quote
siriusdogstar wrote:

From my reading [.....] Asahi Pentax used a 'balanced' circuit, unaffected by battery voltage, in SPII, SPF, ES, and ESII.

[...]

Folks continue to report battery voltage makes a metering difference in their SPII. Maybe some SPII have the balanced circuit and some don't. Serial numbers...


Great idea! Maybe you are onto something (and maybe it will just show that my SPII is behaving weird Wink )

I don't have access to an adjustable power supply. However, I know that my SPII works well with 1.35V and 1.5V. "Working well" defined as giving an identical meter reading with identical light input. So, at both extremes it is OK.

Thr S/N of my SPII is 5351606.

Are Asahi Pentax serial numbers based on the model designation, or do they indicate the production period, regardless of the model?

Wikipedia (yes, non-authoritive) says about the SPII: "... as well as a number of minor improvements to the meter components and film transport,"


PostPosted: Fri Jun 18, 2010 11:40 am    Post subject: Reply with quote
siriusdogstar wrote:
From my reading, the original Spotmatic meter circuit is an 'unbalanced' design wherein battery voltage affects meter reading, however Asahi Pentax used a 'balanced' circuit, unaffected by battery voltage, in SPII, SPF, ES, and ESII.

Then an engineer posted an article online showing how battery voltage does affect meter reading in Asahi's 'balanced' circuit. (I wasn't convinced; need to take another look)

Folks continue to report battery voltage makes a metering difference in their SPII. Maybe some SPII have the balanced circuit and some don't. Serial numbers...

SPII people with access to a good quality bench power supply capable of varying from 1.35 to 1.50 VDC output could power an SPII and tell us if the meter reading changes when voltage is changed. If different folks get different results, the serial numbers list should show which come before & after a circuit change.

It's quite possible Pentax changed the SPII meter circuit design but I've never heard of that before. They did make several minor changes to other things. Can you give a link where you'read about that?

I've had 5 Spotmatics, an SP and four SPIIs, serial numbers:
SP 1316162
SPII 5281325
SPII 5316035
SPII 5317802
SPII 5411561

All of them have the exactly the same meter circuit design as this diagram copied from the Pentax SP service manual. I can't find a manual for the SPII:

Note that the circuit is calculated on a battery output of 1.3v (average over its lifetime) and that the meter requires a current of 3uA to centre the needle. The current is varied by several resistors: the ASA/shutter speed dial (R1 & R2), a calibration resistor (R3) and the CdS cells themselves. When the exposure settings are too high the resistance in the circuit is relatively lower, so the current through the meter is increased and the needle points upwards.

If you introduce a higher voltage battery (1.5v average for silver oxide), the current is increased proportionately by a factor of 1.15 so it will point upwards when the camera exposure settings are correct and needs a higher resistance to correct the current back down to 3uA.


PostPosted: Fri Jun 18, 2010 1:10 pm    Post subject: Reply with quote
peterqd wrote:

If you introduce a higher voltage battery (1.5v average for silver oxide), the current is increased proportionately by a factor of 1.15 so it will point upwards when the camera exposure settings are correct and needs a higher resistance to correct the current back down to 3uA.


Ok, so a 1.5V battery would lead to a 1.15x underexposure? Given that the calibration has not been tampered with?

Would it be likely that the adapter I bought (from the Small Battery Company) is broken, and the supposedly 1.35V is actually 1.5V?

That only would leave the question why the readings from my SPII are in line with my Canon 20D.


Edit: I just took a different camera, a Canon rebel, to compare the readings. I also took two different lenses, and I metered through the window towards my rainy backgarden.

SPII / E387S / 2.8/55 / 200 ASA : 1/500s
Canon Rebel / 2.8/50 / 200 ASA: 1/500s

It might be that the light meter in my SPII is "broken", but it is consistent with two different Canon cameras, and with very different focal lengths.


Another update: I just tested with a Praktica MTL3, and that camera suggests that both the SPII and the Canons overexpose with approx. 0.5 stop. The Praktica definitely has the correct battery, but I don't know how reliable the light meter is. I do trust the Canons though.


PostPosted: Mon Jun 28, 2010 10:23 am    Post subject: Reply with quote
Johan - Is that the Spotty you got from me? I never tested that one but my spotty that is in regular use is a stop out without the same adapter.

SN5063709


PostPosted: Mon Jun 28, 2010 11:45 am    Post subject: Reply with quote
martinsmith99 wrote:
Johan - Is that the Spotty you got from me? I never tested that one but my spotty that is in regular use is a stop out without the same adapter.

SN5063709


Yes, it is. And I'm still very happy with it!
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